1. Field of the Invention
This invention relates generally to the field of leak detection, wherein a leaking fluid causes an impedance change in a sensing cable and the impedance change is detected as an indication of the leak.
2. Related Art
A variety of chemicals (e.g., oils, crude oil, refined petroleum products, gasoline, kerosene, organic solvents, and the like) are stored in underground tanks and transported through buried pipelines. Leakage from these tanks and pipelines can contaminate ground water and cause extensive environmental damage. Further, leaks are difficult to detect and often are not detected until extensive environmental damage has already occurred.
One method of detection has been to run a cable adjacent to the underground pipeline or tank. The cable includes a pair of conductors (e.g., coaxial or twin lead) with a permeable insulation layer disposed therebetween. If a leak occurs, the chemical will permeate the cable and will cause a change in the dielectric properties of the insulation layer such that the impedance of the cable is changed at the point of the leak.
This change in impedance can be sensed and located by time domain reflectometry (TDR) techniques. In TDR, an electrical signal is propagated down the cable, and the cable is monitored for reflections caused by impedance changes in the cable. One difficulty with known TDR systems is that the equipment inherently senses a rise in impedance as the length of the cable increases. This results in a reduced sensitivity to reflections occurring near the far end of the cable.